It is conventionally known that the technique, such as an infrared thermograph, for measuring a heat distribution of the surface of the object noncontactly by picking up the heat image of the object.
Such a noncontact measuring technique for heat distribution has advantages to performing real-time and detailed measurement more easily than the contact measuring method, and to facilitating the maintenance by reducing the failure caused by heat or impact due to the noncontact of the measuring device (especially detecting device such as a sensor).
It is also known that the technique for deciding the quality of the object (determining whether the object is normal or abnormal or whether the object is non-defective or defective) based on the temperature distribution of the surface of the object, and the technique includes (1) picking up a heat image of the reference object (the normal object or non-defective object), (2) picking up a heat image of the measurement object, and comparing the two heat images picked up in (1), (2).
The most convenience technique of comparing two images is to display two images side by side and look for differences, however, it is difficult for a decider to find a slight difference between the images and to insure the decision depending on experiment of the decider.
The other conventional technique of comparing two images is to calculate a “temperature difference” on the basis of temperature information of the pixels of the images for generating a “difference image” which represents the distribution of the temperature difference, and decide the quality in accordance with the difference image.
The conventional technique is, for example, disclosed in Patent Literature 1 (JP 2005-172683 A) and Patent Literature 2 (JP H05-27637 U).
The Patent Literature 1 discloses a technique for picking up the heat image (as the reference) in the initial state of the concrete structure, subsequently picking up the heat image (as the decision object) in the state of heating, and for comparing the two images, thereby deciding the defects of the object, such as the voids, shears, or honeycombs.
However, the technique disclosed in Patent Literature 1 has the case where the position relationship (distance or posture, etc.) between the measurement object and the picking up device (e.g. an infrared camera) may fail to be constant when picking up the images in both states (for example the case where the jig for fixing them is not accurately positioned, where the picking up device moves in response to wind or vibration, where the camera is removed from the fixing position and fixed again), and in that case, it unfortunately becomes difficult to superimpose the two images accurately (to superimpose not to shift the picked-up objects in the both images); as a result, the reliability of the difference image (of the temperature information in the difference image) is lowered.
Especially, if the object moves three-dimensionally against the picking up device, such as the situation that the distance between the object and the device changes, and that the imaging field-of-view of the device rotates around the picking-up direction, whereby superimposing the two images becomes difficult.
Therefore, in order to obtain the high-reliability difference image using the technique of Patent Literature 1, the device (infrared camera) should be fixed immovably to the structure and the object should be kept the constant distance and posture against the device by the jig.
The picking up devices such as the infrared camera must be calibrated regularly, and the camera will be removed from the fixing structure for the calibration. When the device is fixed to the structure after the calibration, the imaging field-of-view may shift from before the calibration, which causes the problem that it is difficult to obtain the high-accuracy difference image.
As mentioned above, the difference image may be only applicable to the case that it is easy to obtain the high-accuracy difference image due to the structures composing the measuring device.
The Patent Literature 2 discloses a technique of real-timely measuring the temperature change of the object, picking up the heat image of the object at “intervals”, generating the difference image between the heat image picked up at the time and that picked up at the time passed the interval, and displaying the difference image.
However, the “intervals” of the Patent Literature 2 seems to be a couple of seconds in consideration of the application thereof, and the interval is based on the estimation that the position relationship between the device and the object would not shift, therefore, if the position relationship is shifted in short time, it becomes to difficult to obtain the high-accuracy difference image.
The technique is also known of picking up the heat image and the image picked up using the visible light of the same field-of-view. For example, disclosed in Patent Literature 3 (JP 2003-270049 A) and Patent Literature 4 (JP H04-283634 A).
In the techniques of the Patent Literatures 3 and 4, the visible light and the infrared light, which are incident from the same field-of-view, are divided into the picking-up elements, for picking up the images at the same time.
However, the techniques of the Patent Literatures 3, 4 are used for securing the accuracy (preventing the shifts) of superimposing the two images by means of picking up the images at the same time. Therefore, it is impossible to superimpose the two images with accuracy, which are picked up at the different time and at the different field-of-view.
The objective of the present invention is to provide a temperature measuring device and a temperature measuring method capable of generating a difference image with high reliability using multiple images, even if the imaging fields-of-view of the images differ from each other.